The present invention relates to a secondary ion mass spectrometer and to a method for the secondary ion mass spectrometric analysis of a sample.
A large number of secondary ion mass spectrometers is known in the prior art. Among said secondary ion mass spectrometers, the time-of-flight secondary ion mass spectrometers (ToF-SIMS) are particularly of interest.
Variant A
A first variant, hereinafter referred to as Variant A, of a ToF-SIMS mass spectrometer, uses short primary ion pulses for generating secondary ions in the sample.
In this variant of ToF-SIMS, the sample is bombarded with very short primary ion pulses in the nanosecond range and the secondary ions generated are accelerated to energies in the keV range. The flight time of the secondary ions is then measured across a distance of a few tens of cm up to a few m. The mass of the secondary ions can be determined from the flight time. The flight time spectrometers used contain ion-optical elements for efficient transport of the secondary ions to the detector and optionally elements for focusing the energy during the flight time. Because of the high extraction voltage, a high proportion of the emitted secondary ions is also detected. The transmission is generally above 50%. Energy focusing by means of ion mirrors (reflectron) or electrostatic sector fields leads to flight times that are largely independent of the starting energy of the secondary ions. As a result, a high mass resolution m/dm of a few 1000 to 10,000 can be achieved.
With typical total flight distances in the range of 1 m, the high acceleration voltages result in travel times of 1 μs to a few 100 μs of the secondary ions depending on the mass. This allows for primary energy pulse frequencies from a few kHz to a few tens of kHz. At this frequency, the sample is bombarded with primary ion pulses and the travel time of the emitted secondary ions is measured. The mass spectra are typically integrated over several cycles. The chemical composition of the sample can be determined from the intensity of the different secondary ions.
For the spatially resolved analysis of small sample regions, the primary ion beam can be focused to a small beam diameter. By means of a suitable deflection device, the primary ion beam can be rasterized over the sample and the mass spectrum can be determined for a larger number of points on the sample (pixels). The lateral distribution of the sample composition can thus be determined (mapping ToF-SIMS). The high pulse frequency of these time-of-flight spectrometers enables a high pixel frequency of the rastering. With typical pixel numbers of 128×128 and 256×256 pixels, a distribution image can be measured in a few seconds.
If the sample is removed by means of the primary ion beam, then the composition of the sample can be measured as a function of the depth z (depth profiling). The combination of mapping ToF-SIMS with sample removal provides the three-dimensional composition of a sample region (3D ToF-SIMS). A 3D measurement with an image stack in the z-direction of a few tens to 100 images can be carried out in a few minutes to a few tens of minutes due to the high pixel frequency and the short image recording times.
The mass resolution of an energy-focusing ToF-SIMS is approximately 10,000 to 16,000. Since the flight time in these devices depends on the sample height, the mass resolution is significantly reduced for rough samples.
Using suitable mass calibration methods, the accuracy of the mass determination is 5-50 ppm. However, it may be reduced considerably for samples with a severe topography. The interpretation of ToF-SIMS spectra can therefore be considerably more difficult for samples having a severe surface topography due to the reduced mass resolution and mass accuracy.
Variant B
In a second variant, hereinafter referred to as Variant B, DC ion beams are used to generate secondary ions.
Instead of pulsing the primary ion beam, the secondary ion beam can also be pulsed. A static secondary ion beam is generated using a DC primary beam.
The energy of this static ion beam is typically in the range of 100 eV using pulsed electrostatic fields, individual ion packets are extracted from this static secondary ion beam and accelerated. This can be done by axial or orthogonal acceleration. The secondary ions are then accelerated to a few keV and their travel time determined in a time of flight analyzer. Such a time-of-flight spectrometer can be operated with frequencies similar to those in Variant A above.
Formation of a focusable secondary ion beam at low energies of approximately 100 eV requires a reduction of the energy width, which can be produced by the desorption process and also by a change in the surface potential in the case of insulators.
This reduction of the energy width can be effected by multipole transfer in combination with a gas collision cooling system. The secondary ions are transported using a multipole with suitable RF voltages and thermalized in a region with high gas pressures by gas collision and collected on the axis of the multipole.
After gas cooling, the secondary ions can be injected into the pulsing unit of the time-of-flight analyzer.
Typically, a portion of up to about 25-30% of the DC secondary ion beam can be utilized for the time-of-flight analysis. This portion decreases at low masses.
The transit time for a secondary ion from the sample to the pulsed extraction is about 5-10 ms. The registration of the secondary ions emitted by a sample site takes, therefore, at least 10 ms. This limits the pixel frequency of such a mapping ToF-SIMS operating in this manner to a maximum of 100 Hz.
Depending on the design and overall flight distance, the mass resolutions of these devices are approximately 5,000 to 50,000. Using suitable mass calibration methods, a mass accuracy of 1-5 ppm is achievable. In contrast to Variant A, the mass resolution and mass accuracy of these devices is not affected by a sample roughness.
The transmission of these time-of-flight mass spectrometers is mass-dependent and is below the transmission of Variant A by a factor of 10 to 100.
Other Mass Spectrometers
Other types of SIMS devices with high mass resolution are known as well. For example, double-focusing magnetic sector fields are also used in conventional SIMS devices with DC primary ion beams. The mass resolution of these devices can be above 10,000. However, a parallel detection of all masses is not possible, but at best the simultaneous detection of a few masses. These mass spectrometers are, therefore, not suited for the analysis of complex organic solids.
For high-resolution mass spectrometry, ion trap mass spectrometers can also be used in the SIMS.
A particularly high mass resolution of over 100,000 is achieved by Fourier Transform Ion Cyclotron Resonance Mass Spectrometers (FTICR). The secondary ions are here injected into a Penning ion trap using a superconducting magnet and stored. After excitation of the ions, their orbital period in the magnetic field can be measured with high accuracy and the mass can be determined therefrom.
The mass resolution of these devices depends largely on the measurement time, The measurement time for a high-resolution mass spectrum is approx. 0.5-5 s. The mass accuracy is at 1-5 ppm.
Other mass analyzers with high mass resolutions are known in mass spectrometry but have not yet been used in the SIMS. For example, Orbitrap™ (Thermo Fischer Scientific Inc., USA) should be mentioned here, which can achieve a mass resolution of over 100,000. The measurement time for a spectrum with the highest mass resolution is approx. 0.5 to 1 s. Reducing the measurement time to 0.05 s is possible, but leads to a simultaneous reduction of the mass resolution by a factor of about 10.
Primary Ion Sources
Various ion sources are known as primary sources for the time-of-flight secondary ion mass spectrometry.
Liquid Metal Ion Sources
For the mapping ToF-SIMS with high lateral resolution, mainly liquid metal ion sources (LMIS) are used. Heavy metal clusters such as those emitted by Bismuth LMIS, for example,
Bi3+, are particularly suited for organic samples. At beam energies of some 10 keV, the DC currents of these ion sources are about 0.1-30 nA with beam diameters of 50 nm up to about 1 μm.
As a result of these high currents, even when generating short ion pulses in the range of nanoseconds, LMISs still provide sufficient primary ion intensities for the ToF-SIMS of Variant A described above.
For primary ions such as Bi3+, the secondary ion yields are very high. However, not only surface molecules are desorbed when penetrating the high-energy primary ions, but also the underlying molecules are destroyed. A high primary ion dose leads to the complete destruction of the organic sample material. Depth profiling and 3D analysis of organic samples is, therefore, not possible with this ion source.
Gas Cluster Ion Sources
Gas clusters with a few 100 to a few 1000 atoms at energies of a few keV to a few 10 keV can be used for the desorption of organic molecules without damage to the underlying material. Typically, Ar or H2O clusters are ionized from a supersonic jet using an electron beam and subsequently accelerated. The gas cluster ion sources (GCIS) typically achieve DC beam currents of 1-10 nA with beam diameters of a few 10 μm. Focusing to a few μm is only possible with extremely low DC currents of a few pA. Beam diameters below 1 μm are not achievable according to the current state of the art. The generation of short pulses of a few ns is difficult due to the broad mass distribution of cluster ions generated by GCIS. At best, pulses of 10-20 ns duration at beam diameters of about 50 μm can be achieved. GCIS are, therefore, not suitable as primary ion sources for ToF-SIMS in Variant A described above.
Dual-beam-SIMS
Another variation of the time-of-flight secondary ion mass spectrometry described above is accomplished by the use of two different primary ion beams.
In ToF-SIMS devices of Variant A described above, a dual-beam method is often used for depth profiling and 3D analysis. A device for carrying out such a dual beam method is shown in FIG. 1.
FIG. 2 shows the time sequence of an analysis. in this case, the surface of a sample (1) is removed for analysis using the ion beam of an analysis ion source (2) as a primary ion beam. The analysis ion source (2) provides short ion pulses for the time-of-flight analysis of the emitted secondary ions by means of a time-of-flight analyzer (5). After extraction of the secondary ions generated by these primary ion beam pulses in an extractor (4), the extraction voltage is switched off and the surface is removed using ion beam sputtering from a sputtering ion source (3). The removal can either take place during the travel time measurement of the secondary ions (interlaced mode, see FIG. 2) or after the end of a analysis cycle (non-interlaced mode). Furthermore, with the extraction field off, the surface of the sample (1) can be irradiated with low-energy electrons to compensate for positive charges on electrically insulating samples. This is usually done with electron energies up to max 20 eV.
By combining the surface analysis by means of primary ions of an LMIS with a removal by gas cluster ions of a GCIS, the accumulation of radiation damage can be avoided. The sample molecules destroyed by the LMIS are removed by the GCIS. Stable signals can be obtained from organic solid samples at suitable relative removal rates of the two ion beams. The typical ratio of the removal rate of the GCIS relative to the LMIS is about 10 to 1000, depending on the sample material. This means that the majority of the sample material is removed by the GCIS with the extraction voltage switched off and therefore does not contribute to the analysis. Combining mapping ToF-SIMS analysis with a high-lateral-resolution using LMIS and removal of appropriate size and energy gas clusters, 3D analysis of organic solids can be performed quickly.
The 3D analysis can be used for the chemical characterization of a wide variety of organic solids. Examples include the 3D analysis of organic LEDs (OLEDs), polymer structures and biological samples such as tissue and single cells.
Problems of 3D ToF-SIMS
In the 3D analysis of organic solids with ToF-SIMS devices of Variant A, the analysis using LMIS allows a high lateral resolution in the sub-μm and μm range. At the same time, the pixel frequency is very high such that a lateral distribution can be measured with a high pixel number and in a short time. For example, the analysis of a 256×256 pixel surface at a typical 10 kHz frequency takes about 6.5 s. A 3D data set with 100 layers in the z direction can be measured in approx. 11 min. However, the interpretation of the data is often very difficult. The mass resolution and mass accuracy of the time-of-flight spectrometers are generally insufficient to reliably identify molecules in a mass range from 100 u to a few 100 u. The initial surface topography and the change of the topography in the course of the measurements due to different removal rates of the different materials in the analyzed volume significantly influences the time-of-flight of the secondary ions. The respective shifts of the peak position in the mass spectrum can lead to errors in the determination of the mass of a molecule. Inaccuracies in mass determination can easily be several hundred ppm. Furthermore, the mass resolution is reduced and the numerous interferences of molecular ions and fragment ions make the detection of molecules in complex organic matrices considerably more difficult.
With ToF-SIMS devices of Variant B, the influence of the topography on mass resolution and mass accuracy is avoided. Thus, the analysis of 3D data is significantly simplified, provided a type of analyzer with a high mass resolution and a high mass accuracy is used. However, there are other problems with this type of device. For one, the pixel frequency is significantly lower for this device type. For example, with a pixel frequency of 50 Hz, a 3D analysis with 256×256 pixels and 100 layers typically takes more than 36 hours. If a mass spectrometer with an extremely high mass resolution and mass accuracy, like an FTICR, is used, the measurement time grows to about 76 days at a pixel frequency of 1 Hz.
On the other hand, the analysis using LMIS with high lateral resolution causes extreme damage to the organic samples. The dose of the DC LMIS beam is already many orders of magnitude above the damage limit of approximately 1E13 primary ions/cm2 when recording an image with a high lateral resolution. A GCIS that avoids this sample damage could be used instead of an LMIS. However, this means that no lateral resolution in the sub-μm range can be achieved. With beam diameters of a few μm, the beam current of the GCIS is already too low to allow a sufficient removal rate for a 3D analysis down to a depth of a few μm.
The objective of the present invention is that of providing a mass spectrometer and a mass spectrometric method that solves the above-mentioned problems that arise both in the case of a ToF-SIMS of Variant A and a ToF-SIMS of Variant B.
This objective, as well as other objectives that will become apparent from the discussion that follows, are achieved in accordance with the present invention, by providing a secondary ion mass spectrometer that comprises:    (a) a first primary ion source for generating a first pulsed primary ion beam with short pulse durations;    (b) a second primary ion source for generating a second pulsed primary ion beam with pulse durations in the range of 50 ns and up to 5 s;    (c) a first TOF-SIMS analysis unit for mass spectroscopic analysis of the secondary ions generated by the primary ion pulses of the first primary ion source from a sample; and    (d) a second analysis unit for mass spectroscopic analysis of the secondary ions generated by the primary ion pulses of the second primary ion source from a sample.
In the case of ToF-SIMS devices of Variant A, in a dual beam analysis of the sample, for example an organic solid, according to the invention a second mass analyzer that is suitable for analysis in the DC mode is used in addition to a time-of-flight mass spectrometer. Advantageously, this analyzer should have the highest possible mass resolution and mass accuracy in order to reliably detect and identify, for example, organic molecules in complex mixtures. The aim is therefore advantageously a mass resolution above 10,000 and a mass accuracy better than 5 ppm.
With this dual beam method, the lateral distribution is now carried out with high spatial resolution by means of a first primary ion beam of a first primary ion source, for example an LMIS, and by means of a ToF-SIMS analyzer in the manner described above for Variant A. In addition, the secondary ions formed during the removal of the sample (in the z-direction) by means of a second ion beam of a second primary ion source, for example a GCIS, are extracted and supplied to a second mass analyzer for analysis in the DC mode.
This combination of a time-of-flight mass analyzer for pulsed secondary ions and such a second mass analyzer now produces an additional high-resolution mass spectrum with high mass accuracy that is not, or is significantly less, affected by the sample topography.
This spectrum is now additionally available for the interpretation of the ToF-SIMS data generated by, for example, an LMIS. In a 3D analysis, a high-resolution mass spectrum can thus be generated additionally for each layer with the second analyzer. These additional high-resolution mass spectra for each layer in the z-direction (direction of removal) alloy for improved identification of the molecules in the 3D data set.
Depending on the maximum pixel frequency of the second mass analyzer, the analysis area can also be subdivided into several fields. Then high-resolution mass spectra with high mass accuracy are available for interpretation for each of these fields.
For a full understanding of the present invention, reference should now be made to the following detailed description of the preferred embodiments of the invention as illustrated in the accompanying drawings.
The solution according to the present invention described above will be described in more detail below with reference to some examples. The same or similar reference signs are used for the same or similar elements and therefore the description may not be repeated.